Carbon nanotube arrays for field electron emission and methods of manufacture and use

ABSTRACT

A method for preparation of carbon nanotubes (CNTs) bundles for use in field emission devices (FEDs) includes forming a plurality of carbon nanotubes on a substrate, contacting the carbon nanotubes with a polymer composition comprising a polymer and a solvent, and removing at least a portion of the solvent so as to form a solid composition from the carbon nanotubes and the polymer to form a carbon nanotube bundle having a base with a periphery, and an elevated central region where, along the periphery of the base, the carbon nanotubes slope toward the central region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 11/381,982 filed on May 5, 2006, which is incorporated hereinby reference.

FIELD OF THE INVENTION

This invention relates to carbon nanotube arrays for use in fieldemission devices (FEDs), and methods of preparation and use thereof.

BACKGROUND OF THE INVENTION

Carbon nanotubes are hexagonal networks of carbon atoms forming seamlesstubes with each end capped with half of a fullerene molecule. They werefirst reported in 1991 by Sumio Iijima who produced multi-layerconcentric tubes or multi-walled carbon nanotubes by evaporating carbonin an arc discharge. Carbon nanotubes (CNTs) have been found to possessexceptional electronic and mechanical properties, making them excellentcandidates for applications relating to nanotube composite materials,nanoelectronics, sensors, and cold electron sources. CNTs can beutilized individually or as an ensemble to build a variety of devices.For instance, individual nanotubes have been used as tips for scanningprobe microscopy and as mechanical nano-tweezers. Ensembles of nanotubeshave been used for field emission based flat-panel displays, and it hasbeen suggested that bulk quantities of nanotubes may be used as ahigh-capacity hydrogen storage media. The electronic behavior of CNTs isdetermined by their structure, i.e., nanotube diameter, length, andchirality. Thus, the precise control of CNT structural elements iscritical for many electronic applications, especially for thedevelopment of field emission devices (FEDs).

FEDs are devices that capitalize on the movement of electrons. A typicalfield emission device includes at least a cathode, emitters, and ananode spaced from the cathode. A voltage is applied between the cathodeand the anode causing electrons to be emitted from the emitters. Theelectrons travel in the direction from the cathode to the anode. Thesedevices can be used in a variety of applications including, but notlimited to, microwave vacuum tube devices, power amplifiers, ion guns,high energy accelerators, free electron lasers, and electronmicroscopes, and in particular, flat panel displays. Flat panel displayscan be used as replacements for conventional cathode ray tubes. Thus,they have applications in television and computer monitors.

SUMMARY OF THE INVENTION

The method of the present invention for fabricating a plurality ofcarbon nanotube bundles hereof comprises:

forming a plurality of carbon nanotubes on a substrate;

contacting the carbon nanotubes with a polymer composition comprising apolymer and a solvent; and

removing at least a portion of the solvent so as to form a solidcomposition from the carbon nanotubes and the polymer to form a carbonnanotube bundle having a base with a periphery, and an elevated centralregion; wherein, along the periphery of the base, the carbon nanotubesslope toward the central region.

The carbon nanotube arrays fabricated using the methods of the presentinvention hereof comprise:

a plurality of carbon nanotubes, the carbon nanotubes disposed on asubstrate, and the bundle having a base with a periphery, and anelevated central region; wherein, along the periphery of the base, thenanotubes slope toward the central region.

in certain embodiments, the carbon nanotube arrays of the presentinvention further comprise,

a carbon nanotube bundle having a first axis substantially perpendicularto a substrate, and

each nanotube in the bundle having a second axis;

the bundle further comprising an outer portion and an inner portion;

the second axis of each nanotube within the outer portion beingsubstantially perpendicular to the first axis, and

the second axis of each nanotube within the inner portion beingsubstantially parallel to the first axis.

The carbon nanotube arrays made in accordance with the methods ofpresent invention can be used in field emission devices (FEDs). The CNTarrays may also be employed in other mechanical and electronicapplications including, but not limited to, the manufacturenanoelectronic sensors, switches, cold cathode ion gauges, and portableX-ray devices.

The field emission devices made in accordance with certain embodimentsof the present invention comprise:

a cathode;

an anode;

a carbon nanotube array disposed on the cathode, the carbon nanotubearray comprising;

-   -   a plurality of carbon nanotube bundles, each bundle comprising a        plurality of carbon nanotubes disposed on a substrate; and

each bundle having a base with a periphery, and an elevated centralregion;

-   -   wherein, along the periphery of the base, the nanotubes slope        toward the central region.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description, which is to be read inassociation with the accompanying drawings, wherein:

FIGS. 1A-1D illustrate schematic cross-sectional views of steps in partof a first embodiment of a process for the formation of a carbonnanotube bundle comprising a plurality of CNTs, according to theinvention;

FIGS. 2A-2D illustrate schematic perspective views of severalembodiments of carbon nanotube bundles, according to the invention;

FIGS. 3A-3F illustrate schematic cross-sectional views of steps in partof a second embodiment of a process for the formation of a carbonnanotube array with a plurality of CNT bundles, wherein the array isfurther patterned with lithographic methods, according to the invention;

FIG. 4 illustrates a schematic cross-sectional view of one embodiment ofa field emission device comprising a carbon nanotube array with aplurality of CNT bundles, according to the invention; and

FIG. 5 shows a plot of the current density vs. macroscopic electricalfield for a PVME treated CNT array with a bundle shape similar to thatillustrated in FIG. 2B (as compared to an identical CNT array nottreated with PVME). The CNT array was 6 mm×6 mm, comprising of aplurality of circular CNT pads, each 100 μm in diameter.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to carbon nanotube (CNT) bundles and arrays ofCNT bundles wherein the surface area at the top of at least one bundlehas been significantly reduced as compared to CNT bundles manufacturedusing conventional methods. The invention also relates to the methods ofmanufacture of the CNT bundles and arrays and to field emission devices(FEDs) comprising the CNT arrays.

Field emission results from the extraction of electrons from a solid bytunneling through the surface potential barrier under the application ofa strong electric field. Above a threshold value and below a saturationvalue, the emitted current is generally dependent on the local electricfield, E, at the emitting surface and the workfunction of the solid, φ.According to the Fowler-Nordheim model, there is a generallyexponential-like dependence of emitted current on the local electricfield and workfunction; as a result, a small variation of the shape orthe environment of the emitter strongly impacts the emitted current.

In many instances, in order for the number of electrons tunnelingthrough the surface barrier to become significant, the electric field isrelatively high, for example, on the order of 3 V/nm or more. To reachsuch a high field, practitioners often take advantage of the fieldamplification effect. Because electric field lines are concentratedaround a sharp object, a decrease in the radius of curvature of anemitter may result in an increase in field amplification.

Carbon nanotubes are excellent candidates for field emission due totheir low work function and high aspect ratios (the length to radiusratio).

Although not wishing to be bound by any particular theory, it is thoughtthat a distance between carbon nanotubes ranging from 1-2 times the tubeheight can increase emitted current density. Practically, however, ithas proven difficult to construct such a CNT array on a substrate due todifficulties relating to the patterning, growth, and intrinsicproperties of CNTs. For example, the high aspect ratio of a CNT makes ita formidable task to grow an individual straight tube vertical to thesubstrate. Therefore, an array consisting of vertically aligned CNTbundles has become a compromise. The utilization of CNT bundles allowsfor relative readiness in emitter construction and enhanced operatingefficiency.

Carbon nanotubes can be produced by methods such as arc-discharge, laserablation or chemical vapor deposition (CVD). The first two methodstypically rely on evaporating carbon atoms from solid carbon sources ata very high temperature. The CVD process involves heating a catalystmaterial to a high temperature in a reactor and flowing a hydrocarbongas through the reactor for a period of time. Several parameters innanotube CVD growth include the hydrocarbon species in the gas, thecatalyst, and the reaction temperature.

In one embodiment, the cathode plane for a cold-cathode field emissiondevice (FED) includes a substrate having a top layer of a material thatemits electrons when subjected to an external electric field. For atleast some FED applications, it is desirable to directly grow the CNTsonto the cathode substrate. One example of a method of generating carbonnanotubes on a surface involves catalytic growth via chemical vapordeposition (CVD). A catalyst layer is evaporated or sputtered on asubstrate, the substrate is loaded into a reaction chamber, and feed gasis introduced at elevated temperatures. Preferably, decomposition of thefeed gas occurs only at the catalyst sites, thus reducing amorphouscarbon generation in the process. Decomposed carbon molecules assembleinto nanotubes at the catalyst nanoparticle sites. Advantageously,catalyst nanoparticles can be patterned on a substrate lithographicallyto realize nanotube growth at intentional locations. For example, thegrowth of nanotubes can be caused to originate at a site of electricalconnections or of mechanical significance. This method results information of bundles of nanotubes with a high population (site) density.

Field emission properties of nanotubes, including the field enhancementfactor and current density, are often dependent on the site density ofthe CNT deposit. These properties may suffer at high densities due to,for example, field screening effects and field penetration. Techniquessuch as e-beam lithography and nanoimprint lithography have beenemployed for exact placement of individual nanotubes that are separatedat pre-determined distances. However, these methods involve use ofsophisticated equipment and are typically costly.

An aligned CNT bundle can be grown with varying sizes on differentsubstrates. Although not wishing to be bound by any particular theory,it is thought that non-bonding interactions between CNTs allow forindividual nanotubes to become associated with one another. As aconsequence, the bundle remains macroscopically vertically alignedthroughout the growth process, resulting in an array composed ofsubstantially vertical CNT bundles. Theoretically, it is desirable togrow CNT bundles of minimal size for use as, for example, FED emitters;however, it has proven difficult to achieve CNT alignment and high yieldwhen the bundle size is small, e.g., a few microns or smaller. In manycases, CNT alignment can become difficult when length of the CNTsbecomes large compared to the bundle size. On the other hand, a largebundle size allows for convenient growth of CNT arrays in high yieldwith a low cost of catalyst patterning on the substrate. Larger CNTbundles of high site density, however, often display field emissionproperties that are not acceptable for use in FEDs.

In accordance with one embodiment of the present invention, a method isprovided to allow for enhanced field emission properties of a CNT bundledue to a reduction of field screening and field penetration effects. Inaddition, the method allows for the fabrication of CNT arrays of highquality growth and in high yield. In certain embodiments, these enhancedproperties are a result of methods that reduce the area at the top of aCNT bundle (i.e., the bundle surface that is distal to the substrate).In accordance with these methods, a CNT bundle is treated with amaterial that significantly reduces the area at the top of the CNTbundles. By reducing the area at the top of the bundle, the fieldamplification factor of the bundle is enhanced. In certain preferredembodiments the CNT bundle is a large bundle with CNTs of high-sitedensity. The size of a CNT can be determined, for example, by alithographic patterning method. In one embodiment, a lower limit of thesize of the CNT bundle is in the range of about 1 to 4 μm². In someembodiments, the lower limit for CNT site density is 10⁸ CNT/cm².

FIGS. 1A-D illustrate a portion of one embodiment of a process whereinan individual CNT bundle is treated with a material that significantlyreduces the area at the top of the bundle. First, carbon nanotubes 102are formed on a substrate 104 as illustrated in FIG. 1B. There are manymethods that can be used to form carbon nanotubes and generally any ofthese methods is suitable. The carbon nanotubes are preferably formed sothat they are generally aligned substantially perpendicular to thesurface of the substrate. Thus, the overall CNT bundle has an axis thatis substantially perpendicular to the substrate. In addition, the carbonnanotubes are, preferably, densely distributed on the substrate. Forexample, the site density of the carbon nanotubes can be at least 1×10⁸CNTs/cm² and, more preferably, at least 1×10⁹ CNTs/cm². Preferably, thecarbon nanotube bundle has an area of from about 20 to about 8000 μm².

In one example of a method for the formation of carbon nanotubes, ann-doped silicon substrate 104 is prepared. As illustrated in FIG. 1A,the top of the substrate 104 is coated with a thin layer 106 of acatalyst suitable for the preparation of the carbon nanotubes.

After the carbon nanotubes 102 are grown, a polymer composition 108 isprovided over the nanotubes, as illustrated in FIG. 1C. This polymercomposition includes at least a polymer and, optionally, a solvent. Insome instances, the polymer may act as its own solvent. In otherinstances a solvent is provided with the polymer. Any method can be usedfor depositing the polymer composition on the carbon nanotubes includingdip coating, spin coating, knife coating, spray coating, and the like.The solvent should not substantially solvate the carbon nanotubes or thesubstrate. Preferably, the polymer composition is sufficiently fluidicto permit uniform coverage of the carbon nanotubes.

Once the polymer composition 108 is disposed over the carbon nanotubes102, the solvent, if present, is at least partially removed, e.g., byevaporation, to produce a solid composition 110 of the carbon nanotubesand polymer, as illustrated in FIG. 1D. Preferably, substantially all ofthe solvent is removed. The structure of the solid composition 110 has abase with a periphery, and an elevated central region, wherein along theperiphery of the base the nanotubes slope toward the central region.

In at least some embodiments, as the solvent is removed by evaporation,at least some of the nanotubes (or the regions of the nanotubes distalto the substrate) are pulled closer together to produce clusters ofnanotubes with, for example, a conical structure when viewed from above.The polymer treatment method encompassed by this embodiment thus resultsin a tighter packing between the nanotubes and reduction in the overallsize of the top of the CNT bundle to, in effect, form a bundle with alarger aspect ratio.

Without wishing to bound by any particular theory, it is thought thatthe shape of the solid composition 110 is a result of the evaporation ofthe polymer or polymer solution from the CNT bundle. The polymer orpolymer solution may form a droplet that encompasses the CNT bundle, andas the droplet evaporates from its edges it acts to bend or pull atleast some of the nanotubes towards the central region of the bundle. Insome instances, based on the physics of evaporation, the nanotubes thatreside in the outer portion of the bundle become substantially parallelwith the substrate and the nanotubes that reside in the inner portion ofthe bundle remain substantially perpendicular to the substrate. Thefinal shape of the solid composition 110 may be affected by the shape ofthe CNT bundle prior to treatment.

In an alternative embodiment, based on the physics of evaporation ofparticular solvents, or based on the size of the solvent droplet ascompared to the bundle, evaporation may occur so as to pull at leastsome of the nanotubes toward the periphery of the base of the nanotubebundle. In this embodiment, at least some of the nanotubes may slopeaway from the central region of the bundle.

In certain embodiments, removal of the solvent may result in the removalof at least a portion of the CNTs from the substrate. In at least somecases, CNT removal by the polymer occurs along the perimeter of thebundle and thus results in a decrease in diameter (or width) of thebase.

For example, as shown in FIGS. 2A-2D, circular bundles and squarebundles can give rise to solid compositions with a conical (FIGS. 2A-2B)or a cylindrical structure (FIG. 2C), and a star structure (FIG. 2D)respectively. The final shape of the solid composition may also beaffected by the ratio of the average height of the nanotubes to theoverall area of the CNT bundle. Accordingly, as this ratio becomeslarger, the outer portion of the CNT bundle wherein the nanotubes aresubstantially parallel to the substrate, in many cases, also becomeslarger. With regard to a circular bundle, for example, as the ratio ofnanotube height to bundle area increases, the shape of the elevatedportion of the treated bundle may change from conical (FIG. 2B) totrapezoidal (FIG. 2C) to cylindrical (FIG. 2D).

Once the solid composition 110 has been formed, at least a portion (andpreferably, all) of the polymer can be removed. In one embodiment, atleast a portion of the polymer is removed by using a solvent thatsolvates the polymer and not the carbon nanotubes. The solvent can bedripped or otherwise poured over the treated CNT bundle or CNT array toremove the polymer or the bundle or array can be placed in the solvent.Upon removal of the at least a portion (and preferably, all) of thepolymer, the nanotube bundle preferably maintains the geometricalstructure associated with solid composition 110.

Additionally, or alternatively, the polymer (for use alone or incombination with the previously described methods), the CNT bundle orarray can be heated in a furnace, preferably, in an inert atmospheresuch as an argon atmosphere. The temperature of the furnace and theperiod for heating can vary. For example, the bundle or array can beheated in an 850.degree. C. furnace for at least 30 minutes to removeresidue of the polymer.

The term “carbon nanotube” refers to a hollow cylindrical articlecomposed primarily of carbon atoms. For example, the nanotubes 102 canhave a narrow dimension (diameter) of about 1-200 nm and a longdimension (length), where the ratio of the long dimension to the narrowdimension, i.e., the aspect ratio, is at least 5. In many CNTs, theaspect ratio is at least 10 and can be 100, 1000, 10,000, 100,000;1,000,000 or more.

When used in the context of a CNT bundle, the axis of the long dimensionof each individual CNT is described in relation to an axis perpendicularto the substrate onto which the CNT bundles arc grown. The longdimension of each CNT can be substantially parallel to the substrate,substantially perpendicular to the substrate, or aligned at any angle inbetween parallel and perpendicular.

The carbon nanotubes of the invention can be either multi-wallednanotubes (MWNTs) or single-walled nanotubes (SWNTs). A MWNT, forexample, includes several concentric nanotubes each having a differentdiameter. Thus, the smallest diameter tube is encapsulated by a largerdiameter tube, which in turn, is encapsulated by another larger diameternanotube. A SWNT, on the other hand, includes only one nanotube.

The terms “carbon nanotube bundle” or “bundle,” as used herein, aresynonymous and refer to a plurality of CNTs that occupy a certain areaof substrate. Bundle size and shape are predetermined by the artisanbased on, for example, the lithographic patterning of catalyst on thesubstrate. The terms “outer portion” or “perimeter” of the bundle, asused herein, are synonymous and refer to an area of the bundlecomprising CNTs that are disposed along the outer edges of the bundle.The terms “inner portion” or “center” of the bundle, as used herein, aresynonymous and refer to an area of the bundle comprising CNTs that aredisposed within the central portion of the bundle. The size of the outerand inner portions are either defined by a width or a diameter dependingon the shape of the CNT bundle. The outer and inner portion of thebundle can either be in direct contact with one another, or separated byan intermediate portion of the CNT bundle of varying width.

In a preferred embodiment, the width of the outer portion is from about5 to about 100 μm, and the width of the inner portion is from about 1 toabout 80 μm. In this context, when a circular bundle is used, thediameter and the width of the inner portion are synonymous.

It will be understood that the term “polymer” includes, but is notlimited, to mixtures or other combinations of polymeric materials, aswell as copolymers and the like. In addition to the polymer and solvent,the polymer composition can also include one or more additives, such assurfactants, plasticizers, antioxidants, filler, tackifiers, organicsolvents, and the like.

Any polymer can be used in the polymer composition 108. Preferably, thepolymer is soft and flexible, not brittle, upon removal of any solvent.Polymers with such characteristics often have a glass transitiontemperature that is no more than 25° C. or room temperature. Morepreferably, the polymer is glassy, tacky, and soft at room temperatureor 25° C., upon removal of the solvent. In addition, the polymer ispreferably soluble in water or an organic solvent that does not solvatethe carbon nanotubes. Examples of suitable polymers include polyvinylmethyl ether (PVME), polyvinyl alcohol (PVA), polyvinylpyrrolidone(PVP), polymethylmethacrylate (PMMA), polystyrene (PS), and the like.

The term “substrate,” as used herein, refers to the structure 104 uponwhich the nanotubes are disposed. In many instances, the substrateprovides the mechanical support for the CNT. Preferably, a portion of orthe entire substrate has a smooth and flat surface that is electricallyconductive and does not react during the carbon nanotube growth process.The substrate can be, for example, a single crystal, polycrystalline,glassy or amorphous material whose surface is the surface on which thenanotubes are deposited or grown. The substrate can comprise one or morelayers that may be structured to form an electronic architecture. Inparticular, an architecture may be constructed which allows each CNTstructure of an array of CNT structures to be separately addressableelectrically. The substrate can also contain a pattern which is eitheruniform or non-uniform. The pattern may include contacts formed andleading to the CNT structures. That is, the substrate may include aplurality of current paths on the substrate, each coupled electricallyto a respective one or more of the CNT structures. The substrate is notcomprised of materials that are reactive with the nanotubes, with anymaterial used in the process for their preparation or with intermediatesformed during the process.

In a preferred embodiment of the present invention, the substrate ismade of semiconductor material such as Si or n-doped silicon, or aninsulating material such as silica, glass, alumina, quartz, ceramicmaterials, mica, a synthetic resin, or graphite. Especially preferred inthe practice of the present invention is n-doped Si in the form of an Siwafer.

The terms “catalyst,” “catalytic metal,” and “metal catalyst,” as usedherein, are synonymous, and refer to any material (e.g., a transitionmetal) that catalyzes the reaction of the carbon-containing feedstock tocarbon nanotubes. Catalytic metals that are suitable for the practice ofthe method invention include, for example, any transition metal,transition metal complex, or transition metal alloy that, when exposedto the reaction chamber and feed gasses, aids in the formation of carbonnanotube structures on the substrate. The catalyst 106 can be depositedon the surface of the non-catalytic interlayer in the form of the activecatalyst or in the form of a pre-catalyst. The pre-catalyst is a metalcontaining material that when treated, for example, by exposure to thehigh temperatures of the reaction chamber, is converted to an activecatalyst capable of promoting CNT growth on the substrate.

In a preferred embodiment of the present invention, the catalyst is atransition metal selected from the group consisting of Fe, Co, Ni, Mo,Pd, and Pt, and complexes and alloys thereof. A particularly preferredmetal catalyst for the practice of the present invention is metallic Fe.

As referred to herein “density” or “site-density” denotes units of CNTstructures per centimeter squared (CNTs/cm²). Site density relates thestatistical spacing distance between individual CNT structures in anarray. For example, a density of about 10⁶ CNTs cm⁻² corresponds to astatistical spacing distance between CNTs of about 10 micrometers (μm).In a preferred embodiment of the present invention, the CNTs disposed onthe substrate have a high site-density.

The term “high site-density,” as used herein, refers to a large numberof CNT structures per centimeter squared (CNTs/cm²). Typically a highsite-density refers to a density of at least about 10⁸ CNTs/cm².

The term “reaction chamber,” as used herein, refers to any apparatusthat provides the reaction conditions for the growth of nanotubestructures. In one embodiment of the present invention, the reactionchamber is a chemical vapor deposition (CVD) apparatus. In one exampleof a CVD process, gaseous mixtures of chemicals are dissociated at hightemperature (for example, CO₂ into C and O₂) and some of the liberatedmolecules may then be deposited on a nearby substrate, with the restpumped away. With regard to the growth of CNTs, the CVD apparatusprovides an atmosphere of a source gas that provides the carbon atomsnecessary for CNT growth. The CVD apparatus may also provide a promotergas and a diluent gas to allow for an enhancement of the purity of thenanotubes grown. Examples of CVD methods include but not limited tothermal CVD, plasma enhanced CVD (PECVD), hot filament CVD (HFCVD), andsynchrotron radiation CVD (SRCVD). In a preferred embodiment of thepresent invention the CVD apparatus is a thermal CVD apparatus.

A thermal CVD apparatus is typically heated to high temperature, e.g.,from about 650 to about 1000° C., to allow for the thermal decompositionof a source gas. Examples of growing nanotubes are discussed below. Itwill be recognized, however, that there are many methods of makingcarbon nanotubes and these methods are, in general, suitable for use inthe present invention. The source gas of the present invention can be,for example, a saturated or unsaturated; linear, branched, or cyclichydrocarbon, or mixture of hydrocarbons, that are gas or vapor phase atthe temperatures at which they are contacted with the catalyst substratematerial (reaction temperature). Other exemplary carbon-containing gasesinclude carbon monoxide, oxygenated hydrocarbons such as acetone andmethanol, aromatic hydrocarbons such as toluene, benzene andnaphthalene, and mixtures of the above. A rate of deposition of carbonon the catalyst material at elevated temperatures will depend on factorsincluding the partial pressures of the carbon-containing gases.Preferred carbon source gases include methane, propane, acetylene,ethylene, benzene, or mixtures thereof. In an especially preferredembodiment, the carbon source gas for the synthesis of low tomedium-site density CNTs is ethylene.

The promoter gas is a substance that is a gaseous compound at thereaction temperatures, and preferably comprises a non-carbon gas such ashydrogen, ammonia, ammonia-nitrogen, hydrogen sulfide, or mixturesthereof. The promoter gas may be useful to reduce the formation unwantedallotropes of carbon, such as graphite, and the deposition of suchmaterials on the substrate surface. The promoter gas of the presentinvention may be diluted by mixing it with a diluent gas, which areprimarily unreactive, oxygen-free gases, such as for example, helium,nitrogen, argon, neon, krypton, xenon, or combinations thereof. For theCVD reaction process of the present invention, hydrogen is preferred forreaction temperatures maintained at less than about 720° C., while forhigher temperatures (greater than or equal to about 720° C.), thepromoter gas is chosen from ammonia, hydrogen, ammonia-nitrogen, or anycombination thereof. The promoter gas can be introduced into thereaction chamber (e.g. a thermal CVD apparatus) at any stage of thereaction process. Preferably, the promoter gas is introduced into thereaction chamber either prior to or simultaneously with the carbonsource gas. In a preferred embodiment, the promoter gas is hydrogen andthe diluent gas is argon.

The methods of the present invention can yield either multi-walled orsingle-walled nanotubes. For promoting multi-walled carbon nanotubegrowth, exemplary CVD methods employ a growth temperature typically inthe range of 650-750° C. with ethylene as the carbon-containing gas.Carbon-containing gases for promoting the growth of single-walled carbonnanotubes include methane, ethylene, acetylene and carbon monoxide. SWNTare usually grown at a temperature in the range of 850-950° C.

The methods of the present invention can provide carbon nanotube arrayswherein the growth of each individual nanotubes is either catalyzed fromthe base of the nanotube, or is catalyzed from the tip of the nanotube.If the mechanism of nanotube growth occurs from the base of the nanotube(catalysis at the catalyst pad surface), the nanotube structure willlikely be attached to the surface of the catalyst pad. However, if themechanism of nanotube growth occurs from the tip of the nanotube(catalysis distal from the catalyst pad surface), the nanotube structurewill likely be attached to the surface of the substrate. Depending onnanotube catalysis factors such as, for example, the identity of thecatalyst (i.e., choice of metal), catalyst pad thickness, growthconditions (e.g., furnace temperature, reaction time), the nanotubes canbe grown from the nanotube base, from the nanotube tip, or both. Incertain embodiments of the present invention the CNT structures will beformed over the exposed surface of the catalytic metal layer. Althoughinitial CNT formation occurs, in most cases, at the catalytic metalsurface, the point of attachment of the CNT (catalyst surface orsubstrate surface) will depend on the mechanism or catalysis (basegrowth or tip growth

The reaction time in the preferred methods of the invention can bevaried depending on the length of the nanotubes desired, with longertimes generally resulting in longer nanotubes. For example, under oneset of reaction conditions, reaction of the substrate in the thermal CVDfurnace for 1 minute will provide CNT arrays wherein the individualnanotubes have an average length of about 30 μm. A 60 minute reactiontime provides for an array wherein the individual nanotubes have alength of about 250 μm. In one embodiment, the preferred reaction timesutilized in the methods of the present invention are from about 1 toabout 10 minutes to provide nanotubes with a length of from about 10 μmto about 70 μm.

Generally, the diameter and length of the carbon nanotubes will dependon the process parameters (e.g., temperature, time, ratio of gases,etc.) and gases used in growing the nanotubes. In addition, somenanotube formation techniques grow single-walled nanotubes and otherstechniques grow multi-walled nanotubes. In one example, multi-walledcarbon nanotubes were grown at 700° C. for 25 minutes on a siliconsubstrate with iron catalyst layer. Different mixtures of gases wereused including a mixture containing 100 sccm (standard cubic centimetersper minute) hydrogen and 690 sccm ethylene and a second mixturecontaining 400 seem hydrogen, 400 sccm ethylene, and 400 sccm argon. Theresulting carbon nanotubes had an average height of about 150micrometers and a diameter in the range of 10 to 40 nm.

In a preferred embodiment, the average diameter of the CNTs in thearrays fabricated using the methods of the present invention is fromabout 10 nm to about 150 nm.

Thus, through the control of both the average height and averagediameter of the CNTs grown from the methods described herein, arrays ofCNTs can be fabricated with the large aspect ratios necessary forelectronic applications such as, for example, field emission devices(FEDs).

In certain embodiments of the present invention, the CNTs are disposedon the substrate as a plurality of CNT bundles to form a CNT array.Preferably, each CNT bundle has a size of from about 20 μm² to about8000μ². The terms “array” and “CNT array,” as used herein, aresynonymous and refer to a plurality of CNT bundles that are attached tothe substrate material proximally to one another. For the purposes ofthe various embodiments of this invention, the CNT array comprises thesubstrate, the plurality of CNT bundles grown thereon, and optionally, aplurality of separate catalyst pads whereupon the CNT bundles are grown.Treatment of the CNT array with a polymer or polymer solution andsubsequent removal of at least a portion of the solvent thus yields aCNT array comprising plurality of solid compositions 110 (See FIG. 1D).The CNT array of the present invention can be used as a cold cathode inthe fabrication of field emission devices and particularly for fieldemission displays.

In a further embodiment of the present invention lies a method forforming patterns of CNT bundles (i.e., a CNT array) on the substrateusing lithographic methods. The lithographic methods described hereinprovide for the precise placement and patterning of catalytic sites ontothe surface of the substrate. Thus, the CNT arrays fabricated using themethods herein can be grown in alignment with, inter alia, current pathson the substrate. In one example, patterns of CNT bundles are grown froma plurality of catalyst pads disposed upon the substrate. The pluralityof catalyst pads can be formed via lithography through the use of, forexample, a resist material.

The terms “resist” and “resist material,” as used herein, are to beunderstood to encompass any material suitable to protect an underlyingsurface during a process treatment. Thus, a resist may be any organic orinorganic chemical substance or compound which can be blanket-depositedand patterned for feature definition. Both positive and negative resistscan be used. The resist can have process selectivity relative to theunderlying material, such as significantly differing etch-rates, or itmay act as a shielding element, for instance, to protect the underlyingsurface from material deposition or ion bombardment. In one embodiment,resist development produces a negative pattern of pads withpre-determined dimensions. These pads can then, for example, be used tocontrol the areas of catalyst deposition on the interlayer to ultimatelycontrol defined areas on the substrate where CNT growth is initiated.

In one embodiment of the present invention, the resist material is aphotoresist material suitable for use with any photolithography method.Photolithography can include masking techniques and other techniques,such as mirrored laser illumination. As one example, a photoresistmaterial is a viscous polymer resin (solution) containing somephotochemically active polymer (PAC), which is typically renderedinsoluble or soluble, relative to a wash solution, by exposure to light.Using a photoresist, a selected pattern can be imaged on a substrate.Areas of a positive photoresist not exposed to electromagnetic radiationmay be removed by a washing process. Alternatively, a negativephotoresist method may be employed, wherein only the areas of thephotoresist material that have been exposed to electromagnetic radiationare removed by washing. As examples, a liquid resist such as is used insemiconductor manufacture or a film resist such is used in themanufacture of printed circuit boards may be used for this purpose.

In a preferred embodiment, the resist material is deposited on thesurface of the substrate prior to the deposition of the catalyst ontothe substrate. The resist layer is aligned with a mask with anappropriate pattern and the exposed areas of the resist layer aretreated with, for example, a UV light source. The resist layer is thendeveloped and the exposed areas removed to provide a pattern of negativepads with pre-determined dimensions, where the substrate surface isexposed. The catalyst layer is then deposited on the exposed substratesurface. Finally, the remaining resist layer is removed through etching.The final product has catalyst pads over a uniform and continuoussubstrate surface.

FIGS. 3A-3F illustrate a portion of one embodiment of a process to forma patterned array of a plurality of CNT bundles using lithography.First, a resist layer 302 is formed on the surface of the substrate 104and the resist layer is patterned using, for example, a mask andexposing the resist layer to, for example, a UV light source. The resistlayer 302 is treated to remove, for example, the exposed portions of theresist layer 304. Next, a catalytic layer is formed on exposed surfaces104(a) of the substrate 104 to form a plurality of catalyst pads 306.The remaining resist layer 302 is then removed and thecatalyst/substrate material is used to form a plurality of CNT bundles102 onto the surface of the plurality of catalyst pads 306.Alternatively, the plurality of CNT bundles 102 can be formed on theplurality of catalyst pads 306 prior to the removal of the remainingresist layer 302. Finally, using the methods set forth herein, the arrayof CNT bundles are treated with a polymer or polymer solution to form anarray of CNT solid compositions 110. Removal of at least a portion ofthe polymer material provides an array of CNT bundles wherein thesurface area of the tops of at least one bundle has been significantlyreduced.

In another embodiment lies a method for the manufacture of a fieldemission device (FED) comprising a CNT-based cold cathode fabricatedusing the methods of the present invention. For example, the CNT arraycomprising CNT bundles of the present invention can be used as the coldcathode. Without wishing to be bound by any particular theory, if thetop area of the CNT bundles is reduced the number of CNTs that emitelectrons under electric field is also minimized, field emissionefficiency can be enhanced. Examples of FEDs include a diode-basedarchitecture consisting of a cathode and an anode; a triode basedarchitecture consisting of a cathode, and anode, and a gate electrode;or a tetrode based architecture further comprising an electrode betweenthe gate electrode and the anode to be used as a focusing grid. The FEDcan be fabricated using methods and materials known in the art, forexample those disclosed in “Materials for Field Emission Displays”(Burden, A. P. International Material Reviews 46:213-231 (2001)) whichis incorporated herein by reference in its entirety

FIG. 4 illustrates a portion of one embodiment for the cathode array ofa field emission device manufactured using certain cold cathode arraysof the invention. The cathode array comprises 1) a cold cathode array,comprising a substrate 104, a plurality of catalyst pads 406, and aplurality of CNT bundles 110; 2) an optional insulator layer 414 on theexposed surface of the interlayer 104; and 3) an optional gate electrodelayer 416 on the exposed surface of the insulator layer 414. The cathodearray of FIG. 4, can then be coupled with an anode using conventionalmethods in the art as, for example, disclosed in “Materials for FieldEmission Displays” (Burden, A. P. International Material Reviews46:213-231 (2001)).

For the manufacture of field emission devices and displays, it is fairlystandard practice to use methods, for example photolithography methods,for forming patterns of the metal cathode and catalyst. Thus, utilizingthe lithography methods set forth herein for the manufacture of FEDs iswell within the scope of the embodiments of the present invention.

In an alternative embodiment, CNT bundles of medium to low site-densitycan be used in the practice of the methods of the present invention.Arrays comprising at least one CNT bundle of medium to low-site densitycan be manufactured, for example, by depositing a non-catalytic metalinterlayer between the substrate and the catalyst as described in U.S.Pat. No. 7,687,981, the entire disclosure of which is incorporated byreference herein in its entirety.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following examples are, therefore, to beconstrued as merely illustrative, and not limitative of the remainder ofthe disclosure in any way whatsoever.

EXAMPLES Materials and Methods

Polyvinyl methyl ether (PVME; 50% aqueous solution) was purchased fromAldrich and directly used without further purification. 1.2% PVME and4.7% solutions were prepared by diluting 50% PVME with deionized waterto the required concentrations. In order to improve its wetability on asurface, a few drops of 10% Triton X100 was added to the resultantsolution.

Silicon wafers were soaked in a Nochromix (from Aldrich) sulfuric acidbath for at least one day to remove contaminants before use.

Photoresist PR-I000A, photoresist developer RD6, and photoresist removerRR4 were products of Futurrex, Inc.

All exemplary CNT bundles and arrays were analyzed by Scanning ElectronMicroscopy (SEM).

Example 1 Lithographic Methods for the Formation of CNT Arrays

Patterning of a silicon wafer followed the sequence of spin-coating aphotoresist on the wafer, thermally treating the treated wafer at 100°C. for 10 min, exposing the treated wafer to UV light through a mask ona Tamarack Mask Aligner, developing the wafer with the Futurrexphotoresist developer to form photoresist patterns on the wafer, rinsingthe wafer with deionized water, blowing the wafer dry with a compressedair stream, evaporating iron (2 to 20 nm, typically about 5 nm) onto thesurface of the wafer using a Semicore E-Beam Evaporator, immersing theiron coated wafer into the Futurrex photoresist remover to dissolve thephotoresist patterns, spraying deionized water onto the wafer to removeany solid residues, and finally blowing compressed air to dry the wafer.In the end, a wafer containing iron patterns was obtained.

Example 2 Lithographic Methods Using a Negative Photomask

A cleaned wafer was evaporated with a thin layer of iron with athickness of about 5 nm on a Semicore E-Beam Evaporator, and thenspin-coated with Futurrex photoresist. After exposure to UV lightthrough a negative mask, the wafer was developed with the Futurrexphotoresist developer. After the developed wafer was rinsed withdeionized water and dried, it was immersed into 0.2% hydrochloric acidto etch off the exposed iron layer, leaving intact the iron layerprotected by the photoresist patterns. The resulting wafer wassubsequently rinsed with deionized water and dried by blowing withcompressed air. The photoresist patterns were removed by dipping thewafer into a Futurrex photoresist remover or acetone orN-methylpyrrolidone. After rinsed with deionized water, the wafer wasdried by blowing it with compressed air stream.

Example 3 Growth of CNT Bundles

The photolithography methods set forth in Examples 1 and 2 providedsilicon wafers patterned with iron catalyst. The silicon wafersconsisted of iron catalyst pads of either circular or square shape. Thediameter of the circular pads were 100 μm or 5 μm and the square padswere 100×100 μm.

The growth of CNT arrays from the patterned iron catalyst describedabove was performed on a FirstNano EasyTube 1000 furnace. The growthtook place at 700° C. in the presence of a mixed gas flow consisting of400-sccm hydrogen, 400-sccm ethylene, and 400-sccm argon. The length ofthe CNTs was controlled by varying growth time.

Example 4 Polymer Treatment of a CNT Array

One method developed provided both for a reduction of the surface areaof the top of the CNT bundles and a reduction of the number of CNTscontributing to field emission. Polymer treatment of an array of CNTbundles was performed by carefully dripping 1.2% PVME solution onto thearray until it was completely covered by the solution. As soon as thearray was in contact with PVME solution, all the CNT bundles wereimmediately soaked up with the solution, evolving large number of tinyair bubbles. The array was placed on a horizontal surface and allowed todry at room temperature. During the drying process, water continuouslyevaporated off, causing the volume of the solution to shrink. When thesolution shrank to the point where the solution could no longer form acontinuous liquid membrane covering up the array, it broke down todroplets, each of which covered up a single CNT bundle. As the solutioncontinued to evaporate, the droplets shrank further toward the center ofthe bundles. The surface tension of the solution pulled the CNTs on theouter portion of the bundle toward the center during this dryingprocess. Drying under these conditions resulted in a bundle wherein theCNTs within the inner portion of the bundle remained substantiallyperpendicular to the substrate while the surrounding CNTs leaned towardit in a radial fashion. The PVME-treated silicon chip containing the CNTarray was then soaked for 15 minutes in deionized water two times toremove PVME from the CNT pattern. The chip was then allowed to dry atroom temperature or dried under a heat gun.

An alternative drying process was also employed, wherein the siliconwafer containing a CNT array was tilted to let the excess PVME solutionto run off of the surface before it was dried in a horizontal positionby blowing mild warm air toward it using a heat gun.

Example 5 Polymer Treatment of CNT Bundles of Varying Height

Examples 5A-C show results of three arrays treated with PVME as setforth in Example 4. Prior to PVME treatment, all of the three arrayswere comprised of circular bundles of 100 μm diameter as described inExample 3.

Example 5A: Using the methods described in Example 3, the patternedsilicon wafer was allowed to react in the furnace for 1 minute to affordan array of CNT bundles of approximately 10 μm in height. After PVMEtreatment, SEM showed that a narrow (−10 μm width) ring was formed bythe collapsed CNTs at the perimeter, which corresponded to the length ofthe CNTs constituting the bundle. A circular plateau (−80 μm diameter)composed of the top portions of the CNTs was surrounded by the ring atthe perimeter. At the center of the plateau was a small tip formed bythe CNTs at the center of the bundle which remained substantiallyparallel to the substrate due to the support from the surrounding CNTs.

Example 5B: Using the methods described in Example 3, the patternedsilicon wafer was allowed to react in the furnace for 3 minutes toafford an array of CNT bundles of approximately 30 μm in height. AfterPVME treatment, SEM showed a ring along the perimeter had increased insize (˜25 μm width) while the circular plateau formed by the top portionof the CNTs significantly decreased in size (−30 μm width) as comparedto example 5A. The tip at the center of the bundle became more prominentand rose significantly above the level of the circular plateau.

Example 5C: Using the methods described in Example 3, the patternedsilicon wafer was allowed to react in the furnace for 10 minutes toafford an array of CNT bundles of approximately 70 μm in height. AfterPVME treatment, SEM showed that the CNTs occupying the inner portion ofthe bundle covered a much smaller area (˜25 μm diameter) as compared toExamples 5A and 5B. In this example, the surrounding CNTs collapsed tothe surface of the substrate to form a ring at the perimeter of ˜35 μmin width.

Patents, patent applications, publications, product descriptions, andprotocols which are cited throughout this application are incorporatedherein by reference in their entireties for all purposes.

The present invention is not limited in scope by the specificembodiments described herein. Indeed, various modifications of theinvention in addition to those described herein will become apparent tothose skilled in the art from the foregoing description. Suchmodifications are intended to fall within the scope of the appendedclaims. It is further to be understood that all values given in theforegoing examples are approximate, and are provided for purposes ofillustration.

1. A carbon nanotube bundle comprising a plurality of carbon nanotubesand polymer disposed at least partially around the carbon nanotubes, thecarbon nanotubes disposed on a substrate, and the bundle having a basewith a periphery, and an elevated central region; wherein, along theperiphery of the base, the nanotubes slope toward the central region. 2.The carbon nanotube bundle of claim 1, the bundle having a first axissubstantially perpendicular to the substrate, and each nanotube having asecond axis; the bundle further comprising an outer portion and an innerportion; the second axis of each nanotube within the outer potion beingsubstantially perpendicular to the first axis, and the second axis ofeach nanotube within the inner portion being substantially parallel tothe first axis.
 3. The carbon nanotube bundle of claim 1, furthercomprising a catalytic metal layer disposed together with the pluralityof carbon nanotubes.
 4. The carbon nanotube bundle of claim 2, whereinthe outer portion and the inner portion are separated by an intermediateportion.
 5. A carbon nanotube array, comprising a plurality of thecarbon nanotube bundles of claim
 3. 6. The carbon nanotube array ofclaim 5, wherein the catalytic metal layer defines a plurality ofseparate pads.
 7. The carbon nanotube array of claim 6, wherein at leastone of the carbon nanotube bundles is disposed on each of the separatepads.
 8. The carbon nanotube array of claim 7, wherein the array furthercomprises a metal cathode in electrical communication with the carbonnanotube bundles and an anode disposed in proximity to the carbonnanotubes.
 9. A method of making a carbon nanotube bundle, which methodcomprises: forming a plurality of carbon nanotubes on a substrate;contacting the carbon nanotubes with a polymer composition comprising apolymer and a solvent; and removing at least a portion of the solvent soas to form a solid composition from the carbon nanotubes and the polymerto form a carbon nanotube bundle having a base with a periphery, and anelevated central region; wherein, along the periphery of the base, thecarbon nanotubes slope toward the central region.
 10. The method ofclaim 9, further comprising removing at least a portion of the polymerfrom the bundle.
 11. The method of claim 10, wherein removing at least aportion of the polymer from the bundle comprises removing substantiallyall of the polymer from the bundle.
 12. The method of claim 9, whereinforming a solid composition comprises removing a majority of the solventfrom the polymer composition.
 13. The method of claim 9, wherein forminga solid composition comprises removing substantially all of the solventfrom the polymer composition.
 14. The method of claim 9, wherein thesolvent is removed by evaporation.
 15. The method of claim 9, whereinthe glass transition point of the polymer is no more than 25° C.
 16. Themethod of claim 15, wherein the polymer is polyvinyl methyl ether(PVME).
 17. A field emission device, comprising: a cathode; an anode; acarbon nanotube array disposed on the cathode, the carbon nanotube arraycomprising; a plurality of carbon nanotube bundles, each bundlecomprising a plurality of carbon nanotubes disposed on a substrate; andeach bundle having a base with a periphery, and an elevated centralregion; wherein, along the periphery of the base, the nanotubes slopetoward the central region and, at the elevated central region, thecarbon nanotubes form a star-shaped plateau with at least four points.18. The field emission device of claim 17, further comprising acatalytic metal layer between the substrate and the plurality of carbonnanotubes.
 19. The field emission device of claim 18, wherein thecatalytic metal layer defines a plurality of separate pads.
 20. Thefield emission device of claim 19, wherein at least one of the carbonnanotube bundles is disposed on each of the separate pads.